System for transmitting traction and braking forces in a rail vehicle

ABSTRACT

The running gear and vehicle body have a linkage connecting each to transmit traction and braking forces in a smooth kinetically accurate manner. The linkage includes a pair of draw bars which are pivotally connected to either the body or running gear and are longitudinally inclined to the horizontal. These bars are interconnected together at their opposite ends by a coupling means. The coupling means can be in the form of an assembly including a connecting rod and a pair of levers; a connecting rod, guide arm and a pair of levers; a single pivotally mounted connecting rod; or a pair of fluid pressure cylinders which interact with pistons on the ends of the draw bars.

United States Patent i191 Kreissig SYSTEM FOR TRANSMITTING TRACTION AND BRAKING FORCES IN A RAIL VEHICLE [75] Inventor: Ernst Florian Kreissig, Seuzach,

Switzerland [73} Assignee: Schweizerische Lokomotiv-und Maschinenfabrik, Zurich, Switzerland 22 Filed: Sept. 25, 1972 21 App]. No.: 291,933

30 Foreign Application Priority Data Sept. 24, 1971 Switzerland 13967/71 [52] US. Cl. 105/165, 188/190 [51] Int. Cl B6lf 5/24 [58] Field of Search 105/165, 168; 188/33, 56, 188/190-194 [56] References Cited UNITED STATES PATENTS 205,956 7/1878 Hamilton et a1. 105/165 1,138,357 5/1915 Curwen 105/165 X 1,307,326 6/1919 Thomas 105/168 1 Aug. 13, 1974 2,199,128 4/1940 Crossman 188/190 3,528,374 9/1970 Wickens 1. 105/165 X FOREIGN PATENTS OR APPLICATIONS 837,711 5/1952 Germany 105/165 Primary Examiner-Duane A. Reger Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin ABSTRACT The running gear and vehicle body have a linkage connecting each to transmit traction and braking forces in a smooth kinetically accurate manner. The

. linkage includes a pair of draw bars which are pivotally connected to either the body or running gear and are longitudinally inclined to the horizontal. These bars are interconnected together at their opposite ends by a coupling means.

The coupling means can be in the form of an assembly including a connecting rod and a pair of levers; a connecting rod, guide arm and a pair of levers; a single pivotally mounted connecting rod; or a pair of fluid pressure cylinders which interact with pistons on the ends of the draw bars.

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SYSTEM FOR TRANSMITTING TRACTION AND BRAKING FORCES IN A RAIL VEHICLE This invention relates to a system for transmitting traction and braking forces in a rail vehicle. More par ticularly, this invention relates to a linkage for accurately transmitting traction and braking forces in a rail vehicle.

Various systems have been known for transmitting traction and braking forces in a rail vehicle between a body of the vehicle and a running gear of the vehicle which supports the body. One known system utilizes a linkage having two consecutively disposed drawbars one to the body and the other to the running gear. Each drawbar is subdivided into two parts which are interconnected by a pivot having a transverse pivot axis and which can pivot relative to one another only in the vertical plane. During use, should such an arrangement be subjected to horizontal forces which arise in vertical springing motions of the body, the horizontal forces can be transmitted to the running gear via the drawbars in such a way as to cause jerking. Thus, in order to re duce this jerking effect, such a linkage has generally been provided with devices such as spring elements to absorb or damp out the jerking forces. However, in order to operate effectively, the spring elements require accurate adjustment or prestressing, supervision and possibly readjustment.

Accordingly, it is an object of the invention to effect a smooth and kinetically accurate manner of transmitting traction and braking forces in a rail vehicle.

It is another object of the invention to provide a linkage for smoothing out jerking effects on a rail vehicle which requires little supervision.

It is another object of the invention to provide a linkage for smoothing out jerking effects in a rail vehicle which can be employed within a small space.

It is another object of the invention to provide a linkage which can be easily incorporated in existing rail vehicles.

Briefly, the invention provides a rail vehicle having a body and a running gear truck or chassis which supports the body with a linkage between the body and running gear. The linkage includes a pair of draw bars which are each inclined towards each other longitudinally to a horizontal plane while being pivotally connected at one end to the body or running gear as well as a coupling means which interconnects the draw bars together at the opposite ends. The coupling means is connected to the other of the body or running gear.

In one embodiment, the coupling means includes a pair of levers, such as bell cranks or bent levers, each having a pair of arms and a connecting rod. Each lever is pivotally mounted to the body or running gear, as the case may be, and is pivotally connected via one arm to a draw bar and via the other arm to the connecting rod. The arms of the levers which connect to the rod are disposed in parallel relation while the other arms are disposed centrally and symmetrically of the running gear. This coupling means can be connected to the underside of the running gear, resulting in a long draw bar, or to the topside of the running gear, resulting in a shorter draw bar. Also, the draw bars can be located on the longitudinal axis of the running gear or to the sides of the running gear.

In another embodiment, the coupling means includes a pair of levers, as above, mounted to the same side of the running gear relative to the transverse axis of the running gear as well as a connecting rod interconnecting the levers and a guide arm interconnecting one of the levers with a drawbar.

In still another embodiment, two pairs of drawbars are used to transmit the various forces. In this embodiment, each pair of drawbars are connected to a coupling means similar to the above. Also, the pairs of drawbars are articulated so that one pair acts only in one direction of travel of the rail vehicle while the other pair acts only in the other direction of travel.

In still another embodiment, the coupling means for connecting the drawbars is constituted by a single lever which is pivotably mounted in a horizontal plane about a vertical axis.

In another embodiment, the coupling means includes fluid pressure cylinders which slidably receive the ends of the drawbars and are interconnected to each other to smooth out veritcal springing of the rail vehicle.

A particular advantage of the system according to the invention is that the linkage and coupling means can be positioned in a way best suited to the restricted space available in running gear so that, for instance, all the space in the center of the running gear is available'to receive various items of traction equipment.

In the form of the invention in which the coupling means has at least one lever pivoted to the running gear or to the body is distinguished by great simplicity such that it can be produced and, more particularly, fitted to a rail vehicle at very reduced cost.

Where the lever is disposed transversely and for pivoting around a substantially upright pivot, the drawbars can each be disposed in virtually any arbitrarily selected zone of vehicle width so that forces can be transmitted positively independently of the height of the coupling device.

Where the pivots and those arms of the bell-cranksv which are connected to the drawbars are disposed centrally and symmetrically in relation to the running gear and the lever arms which are interconnected by the connecting rod in parallel relation, there is little stressing of components.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates an elevational view of a rail vehicle incorporating a linkage according to the invention therein;

FIG, 2 illustrates a part plan view of the linkage of FIG. 1;

FIG. 3 illustrates an elevational view of a rail vehicle incorporating a modified linkage similar to that shown in FIGS. 1 and 2;

FIG. 4 illustrates a plan view of the linkage of FIG.

FIG. 5 illustrates a view similar to FIG. 1 of a linkage wherein a pair of draw bars are mounted on the longitu- FIG. 9 illustrates a view similar to FIG. 7 wherein the linkage is connected to the top of the running gear;

FIG. 10 illustrates a plan view of the linkage of FIG. 9;

FIG. 11 illustrates a view similar to FIG. 1 wherein a pair of linkages are connected between a vehicle body and a running gear;

FIG. 12 illustrates a plan view of the linkage of FIG. 11;

FIG. 13 illustrates a view of a modified linkage similar to that of FIGS. 3 and 4;

FIG. 14 illustrates a plan view of the linkage of FIG. 13;

FIG. 15 illustrates a view of a linkage similar to that shown in FIGS. 11 and 12;

FIG. 16 illustrates a plan view of the linkage of FIG. 15;

FIG. 17 illustrates a view similar to FIG. 1 of a linkage utilizing a single horizontally disposed lever;

FIG. 18 illustrates a plan view of the linkage of FIG. 17;

FIG. 19 illustrates a view similar to FIG. 1 of a linkage utilizing a fluid pressure cylinder; and

FIG. 20 illustrates a plan view of the linkage of FIG. 19.

For simplicity, like reference characters are used throughout the drawings to indicate like parts.

Referring to FIG. 1, a rail-vehicle body 1, only some of which is shown, is supported on a running gear or chassis 2 by way of two springs 9, only one of which can be seen. The springs 9 are disposed on spring carriers 10 which are rockingly suspended at the sides of the frame of the running gear 2. The running gear 2 comprises two sets of wheels 3, 4 each of which are journalled in two bearings 5 rockably mounted on the running-gear frame. Disposed on each bearing 5 are two resilient support members 6 on which the running gear frame rests.

Referring to FIGS. 1 and 2, each wheel set 3, 4 is driven by a motor 7 through a suitable known gearing 8. Each motor 7 has a housing from which various brackets project to mount the motor 7 in the running gear frame. One bracket 50 projects from one side while two brackets 51 project from an opposite side. The brackets 50 of the two motors 7 extend towards one another and are screwed together and the brackets SI are screwed to the running-gear frame. The motor housings are each connected via a connecting passage 52 to the deliveries of a cooling fan (not shown).

Referring to FIG. 1, each of the wheels of the sets 3, 4 is fitted with clasp brakes 53 whose shoes 54, 54' are operated by way of linkage 56 and of a brake cylinder 55 disposed on the running-gear frame. The brake cylinders 55 are connected in known manner to a control facility (not shown).

As shown in FIGS. 1 and 2, the body 1 and running gear 2 are interconnected by a linkage comprising two drawbars or traction rods ll, 12 which extend lengthwise of the running-gear frame on both sides thereof. The two drawbars ll, 12 are each connected, for pivoting around a horizontal axis, to two dependent members 13 on the bottom of the body 1; as considered in the direction of travel, one pair of members 13 is in front of the running gear and the other pair is behind the running gear. Outside the running-gear frame the two bars ll, 12 so incline downwardly in and against the direction oftravel towards the transverse centerplane E of the running gear 2 that, seen from the side, the imaginary extended longitudinal axes of the bars 1 l, 12 cross at rail-top height S in the transverse center plane E which is near the region where the body is supported, i.e., near the region of the axis of running-gear rotation. This arrangement, in which the theoretical point of intersection of the forces acting on the running gear 2 is at rail-top height, provides optimum stressing of all components.

The two bottom ends of the bars 11, 12 are interconnected by a coupling means 14 which is positioned horizontally a short distance above the ground and which comprises two double-arm levers l5, 16 so interconnected by a transverse connecting rod 17 as to be pivotable to the same hand of rotation. The two levers l5, 16 are bell cranks or bent levers which are pivoted to the running gear 2, i.e., in the construction shown, to the undersides of the motor housings via two vertical pins 18 which are arranged centrally and symmetrically in relation to the running gear 2. The arms of the bell crank 15 include an obtuse angle a and the arms of the bell crank 16 include an acute angle B, the angle a and B being supplementary, i.e., a B

The bell cranks 15, 16 have parallel arms, the rod 17 being pivoted to those two arms of the levers 15, 16 which are near one another. When the bell cranks 15, 16 are in the operative position shown, their other arms, assuming that the vehicle is stationary or travelling straight ahead, extend parallel to the wheel axles. The draw bars 11, 12 are each so connected to the ends of the laterally projecting arms of the bell cranks l5, 16 as to be able to make appropriate transverse pivoting motions when the body 1 pivots relatively to the running gear 2, as when the vehicle negotiates curves.

The draw bars ll, 12 and the coupling means 14 provide a kinetically accurate transmission of traction forces outside the space between the motors. Thus, this space is available to receive the various parts which have to be placed in the running gear 2. In the event of vertical springing movements of the body 1, the corotating bell cranks 15, 16 provide smooth equilization of the horizontal components of the traction forces.

Referring to FIGS. 3 and 4, the system shown is of similar construction to the system shown in FIGS. 1 and 2 except that two draw bars 11, 12 and a coupling means 14 interconnecting the draw bars ll, 12' are disposed between the running gear 2 and the bottom of the body 1. In this embodiment, the bottom of the body 1 is higher than in the embodiment shown in FIGS. 1 and 2. The bell-crank pivot pins 18 are disposed centrally and symmetrically on the top of the running-gear frame. The arrangement shown, in which the longitudinal axes of the bars 11', 12' intersect one another at rail-top height S, leads to short bars ll, 12 and to a relatively long connecting rod 17'.

Referring to FIGS. 5 and 6, the draw bars 11, 12 are disposed between the wheels near the running-gear longitudinal center plane L. The pairs of dependent members 13 are each secured to a part of the body 1 at the same level as the running-gear frame. So that they may both pivot to the same hand, the bell-cranks 15, 16 are the other way round as compared with the embodiment of FIGS. 1 and 2. Consequently, as shown in FIG. 6, the rod 17 crosses that arm of bell-crank 16 which is connected to bar 11. correspondingly, rod 17 is pivotally connected to the undersides of the bell cranks 15, 16. This construction is of use more particularly for rail vehicles having body parts which come down low nearv the running gear 2.

Referring to FIGS. 7 and 8, the relationship between the position of body 1 and the running gear 2 is very similar to the case shown in FIGS. 3 and 4. The bars 1 l, 12 are interconnected by a coupling means 19 disposed horizontally on the underside of the running-gear frame. The coupling means 19 comprises two levers (bell-cranks or bent levers) 21, 21 which are interconnected by a transverse rod 20 and whose arm are at 90 to one another. The coupling means 19 also comprises a longitudinally extending connecting rod 22 and a guide arm 23. The two bell-cranks 21, 21 are mounted near the wheel set 4 on vertical pins 18 which are disposed on the same side of the running gear symmetrical of the running-gear longitudinal center plane L and which are disposed on brackets on the underside of the running-gear frame. Adjacent arms of the levers 21, 21 are parallel to one another and coupled together for pivoting in the same hand by the connecting rod 20, the rod 20 extending parallel to the wheel axles. The other arms of the levers 21, 21' project beyond the sides of the running gear 2. Draw bar 12 is pivoted to the arm of lever 21 and rod 22 to the corresponding arm of lever 21'; the other end of rod 22 is bifurcated and piv oted, together with the end of bar 11, to guide arm 23. Near the wheel set 3, the guide arm 23 is mounted on a vertical pin 18 disposed on the underside of the running-gear frame.

The fact that the rod 20 extends parallel to the wheel axle enables the linkage system to be of very compact construction. Although the rod 20 extends through the space between the two motors 7 there is very little reduction of such space.

Referring to FIGS. 9 and 10, the draw bars 11, 12 are pivoted to brackets 29 on the body 1 and interconnected by a coupling means 24 similar to the one used in the embodiment of FIGS. 7 and 8. The difference from the latter embodiment is that the rod 20 interconnects two levers 25, 25, each having two arms at rightangles to one another, the arms being disposed in spaced-apart vertical relationship on a common pivot 26. In the example shown, the levers 25, 25' are mounted by way of theirvertical pivots 26 in the running-gear frame side bearers. Those arms of the levers 25, 25 which are interconnected by the rod 20 are disposed above the running-gear frame, while those arms of the levers 25, 25' which are connected to the rod 22 or bar 12 are disposed below the running gear frame. The arrangement of the parts which form the linkage 24 in planes disposed one above another enable traction and braking forces to be transmitted substantially independently of running-gear construction.

Referring to FIGS. 11 and 12, two pairs of draw bars 27, 27' and 28, 28' are pivoted to the vehicle body 1. In relation to the direction of travel, indicated by an arrow 30, the draw bar pair 27, 27 is connected to the back end of the running-gear frame while the draw bar pair 28, 28 is connected to the front end of the frame. Each bar pair is interconnected by a coupling means 31 which, as in the embodiment of FIGS. 7 and 8, comprises two bell-cranks 21, 21' pivoted to the underside of the vehicle frame, plus a connecting rod 20. The draw bar ends connected to the levers 21, 21 are formed with slots 32, 33 (FIG. 11) each so engaged by a guide pin on the corresponding lever arm that the guide pin and the draw bar can moverelative to one another lengthwise. When the running gear 2 is in the position which is shown in FIG. 1 1 and which corresponds to the direction of travel indicated by the arrow 30, the guide pins of the rear levers 21, 21 contact the front ends of the slots 32 in the bars 27, 27 so that the bars 27, 27 can transmit traction forces to'the body 1. The bar pair 28, 28', on the other hand, is inoperative since the guide pins of the front levers 21, 21' take up a central position in the slots 33 in the bars 28, 28'. Correspondingly, in the opposite direction of travel the pins of the lever arms 21, 21' move up against the ends of the slots 33, whereas the guide pins in the slots 32 take up a central position. The example shown is of a construction having a body part coming very low above the running gear 2. By arranging the coupling means 31 at the two outermost ends of the running-gear frame, the whole space associated with the running gear 2 is made available for other fitments.

Referring to FIGS. 13 and 14, the arrangement of the linkage interconnecting the body 1 and running gear 2 is basically the reverse of the arrangement shown in FIGS. 3 and 4. Each of the draw bars 11, 12 is pivoted to a horizontal pin 34 projecting from the running-gear frame and to one of the two levers 15, 16. Each lever l5, 16 is mounted on a vertical pin 18 disposed at the bottom of the body 1. This system operates very similarly to the system shown in FIGS. 3 and 4. 1

Referring to FIGS. 15 and 16, the arrangement of the linkage interconnecting the body 1 and running gear 2 is basically the reverse of that shown in FIGS. 11 and 12, except that in FIGS. 15 and 16 each of draw bars 27, 27; 28, 28' is pivoted to a pin 34 on the runninggear frame while each of the levers 21, 21 of the coupling means 31 which interconnects the other ends of each draw bar pair is mounted on a pin 18 disposed on the underside of the vehicle body 1.

Referring to FIGS. 17 and 18, the body 1 is mounted on running gear 35 whose side bearers are interconnected by a transverse cross bearer 36 disposed near the transverse center plane E. The manner in which the body 1 is borne by the running gear 35 is not shown. The motors 7 are screwed by means of brackets 51 to the edge bearers of the running-gear frame and at their other ends are suspended on one each of rockers 37 pivoted to the cross-bearer 36. Each draw bar 11, 12 is pivoted on a horizontal pivot 38 disposed on the body 1. A coupling means 39 is embodied by a horizontal inclined two-armed lever (equalizer) 40, and the bottom ends of the bars 11, 12 are pivoted to the ends of the lever 40. The lever 40 is mounted by means of a vertical pivot 41 in a bearing 42 disposed on the crossbearer 36. When the body 1 makes a vertical stroke relative to the running gear 35, movements of the bars 1 l, 12 are compensated for by pivoting of the lever 40. Transmission of traction and braking forces is not impaired.

Referring fmally to FIGS. 19 and 20, two draw bars 43, 44 are pivoted to the two side bearers of the running gear 2 by way of the horizontal pins 34, and a piston 45 is secured to the free end of each bar 43, 44. The two pistons 45 are guided in seal-tight manner in two fluid pressure cylinders 46, 47 of a hydraulic coupling means 48, the cylinders 46, 47 being pivoted to dependent members 13 on the bottom of the body 1. Each cylinder 46, 47 is closed by an end member through which the associated bar 43, 44 extends in seal-tight manner. The working chambers of each of the two cylinders 46, 47 are separated by the pistons 45 and interconnected in crossover-fashion by way of lines 49a, 49b i.e., the working chamber above piston 45 of one cylinder 46 or 47 is connected to the working chamber below the piston 45 of the other cylinder 47, 46, respectively.

When the body 1 makes a vertical springing movement relative to the running gear 2, the pistons 45 move correspondingly in their cylinders 46, 47, displacing hydraulic fluid, e.g.. from the bottom two chambers of the cylinders 46, 47 and causing a corresponding quantity of fluid to enter the top chamber of the other cylinder 47 or 46. A positive connection is therefore provided between the two draw bars 43, 44 thus insuring smooth transmission of traction and braking forces.

Other embodiments of the system according to the invention are of course possible. For instance, the pressure cylinders can be pivoted to the running gear and the draw bars to the vehicle body. Also, a pneumatic coupling means or a coupling means actuated in some other way, possibly by a servomotor, can be used.

Also feasible is a system comprising a skew coupling linkage having two bent levers each disposed in a plane corresponding to the inclination of the associated draw bar and being mounted for pivoting on a pin disposed perpendicularly to the plane of the draw bar. In this case. an inclined connecting rod must be used to interconnect the two adjacent arms of the bent levers. This feature reduces stressing of the bent levers by vertical components of the forces introduced via the draw bars.

What is claimed is:

1. In combination with a rail vehicle having a body and running gear for supporting said body thereon while transmitting traction and braking forces to said body, said running gear including a frame and at least one set of wheels journalled in said frame; at least one linkage between said body and said running gear comprising a pair of draw bars, each said bar being inclined longitudinally to a horizontal plane and pivotally connected at one end to one of said body and said running gear, and

coupling means interconnecting said draw bars together, each said bar being connected at an opposite end to said one end to said coupling means and said coupling means being connected to the other of said body and said running gear.

2. The combination as set forth in claim 1 wherein said coupling means includes at least one lever pivotally mounted on said other of said body and said running gear.

3. The combination as set forth in claim 2 wherein said lever is disposed in a horizontal plane and which further includes a vertically disposed pivot mounted on said other of said body and said running gear and pivotally mounting said lever thereon.

4. The combination as set forth in claim 1 wherein said coupling means includes a pair of bell cranks. each respective bell crank being connected to a respective one of said draw bars and pivotally connected to said other of said body and said running gear; and a rod interconnecting said bell cranks for pivoting in the same direction of rotation.

5. The combination as set forth in claim 4 which further includes a pair of pivots mounted on said other of said body and said running gear, each pivot pivotally mounting a respective bell crank thereon, and wherein each bell crank has a pair of cams, one of said arms being connected to said rod and the other of said arms being connected to a respective draw bar; said arms of said bell cranks connected to said rod being parallel to each other and said other arms being disposed centrally and symmetrically to said running gear.

6. The combination as set forth in claim 1 wherein said coupling means includes a pair of pistons, each respective piston being connected to a respective draw bar, and a pair of interconnected fluid pressure cylinders, each respective piston being slidably mounted in a respective cylinder.

7. The combination as set forth in claim 1 wherein said draw bars are inclined downwardly in opposite directions to each other relative to the direction of travel of said vehicle body to form an angle having an apex disposed in the central transverse plane of said running gear approximately at rail top height.

8. ln combination with a rail vehicle having a body and a chassis for supporting said body while transmitting traction and braking forces to said body; at least one linkage between said body and said chassis providing a kinetically accurate transmission of traction forces therebetween comprising:

a pair of draw bars, each said bar being inclined longitudinally to a horizontal plane and pivotally connected at one end to said body; and

coupling means interconnecting said draw bars together, each said bar being connected at an end opposite to said one end to said coupling means and said coupling means being connected to said chasslS. 

1. In combination with a rail vehicle having a body and running gear for supporting said body thereon while transmitting traction and braking forces to said body, said running gear including a frame and at least one set of wheels journalled in said frame; at least one linkage between said body and said running gear comprising a pair of draw bars, each said bar being iNclined longitudinally to a horizontal plane and pivotally connected at one end to one of said body and said running gear, and coupling means interconnecting said draw bars together, each said bar being connected at an opposite end to said one end to said coupling means and said coupling means being connected to the other of said body and said running gear.
 2. The combination as set forth in claim 1 wherein said coupling means includes at least one lever pivotally mounted on said other of said body and said running gear.
 3. The combination as set forth in claim 2 wherein said lever is disposed in a horizontal plane and which further includes a vertically disposed pivot mounted on said other of said body and said running gear and pivotally mounting said lever thereon.
 4. The combination as set forth in claim 1 wherein said coupling means includes a pair of bell cranks, each respective bell crank being connected to a respective one of said draw bars and pivotally connected to said other of said body and said running gear; and a rod interconnecting said bell cranks for pivoting in the same direction of rotation.
 5. The combination as set forth in claim 4 which further includes a pair of pivots mounted on said other of said body and said running gear, each pivot pivotally mounting a respective bell crank thereon, and wherein each bell crank has a pair of cams, one of said arms being connected to said rod and the other of said arms being connected to a respective draw bar; said arms of said bell cranks connected to said rod being parallel to each other and said other arms being disposed centrally and symmetrically to said running gear.
 6. The combination as set forth in claim 1 wherein said coupling means includes a pair of pistons, each respective piston being connected to a respective draw bar, and a pair of interconnected fluid pressure cylinders, each respective piston being slidably mounted in a respective cylinder.
 7. The combination as set forth in claim 1 wherein said draw bars are inclined downwardly in opposite directions to each other relative to the direction of travel of said vehicle body to form an angle having an apex disposed in the central transverse plane of said running gear approximately at rail top height.
 8. In combination with a rail vehicle having a body and a chassis for supporting said body while transmitting traction and braking forces to said body; at least one linkage between said body and said chassis providing a kinetically accurate transmission of traction forces therebetween comprising: a pair of draw bars, each said bar being inclined longitudinally to a horizontal plane and pivotally connected at one end to said body; and coupling means interconnecting said draw bars together, each said bar being connected at an end opposite to said one end to said coupling means and said coupling means being connected to said chassis. 